Port-Orford-Cedar Root Disease

Phytophthora lateralis

Tucker & Milbrath

Last updated by:

Faith Campbell

August 2004

Port-Orford-cedar, Chamaecyparis lawsoniana, is a valuable forest tree that has a limited distribution along the Pacific coast from southern Oregon to northern California. The species has highly aromatic wood and is widely used as a landscape plant. Port-Orford-cedar populations have been heavily damaged by a root disease caused by the exotic algal fungus Phytophthora lateralis. The disease was first reported in 1923 near a Seattle nursery (Hunt in Zobel et al., 1985), which is outside Port-Orford-cedar’s natural range. Roth et al. (1987) reported that in 1938, the disease was again discovered in Oregon’s Willamette Valley, where it virtually halted nursery production of ornamental Chamaecyparis. The disease was discovered in the naturally occurring Port-Orford-cedars in 1952 (Roth et al., 1987) and has since spread throughout the host’s range. How Phytophthora lateralis entered the country or even the exact country of origin is uncertain. Partial resistance in Asian Chamaecyparis species suggests an eastern Asian origin (Roth et al., 1987).

The fungus invades fine roots and subsequently colonizes the entire root system. Multiple root infections will spread to the root collar, and the tree will die from girdling. The fungus will attack seedlings as well as mature trees. Seedling mortality can occur within a few days, while mature trees may take two to four years to die. Aerial infections of branches that come into contact with infected litter or soil can occur during wet weather. The infection will eventually migrate to the trunk, and girdling will occur. Fungal spores are spread by surface water or rain splash, usually in the wet spring. The pathogen can be transferred in spore-contaminated soil by machinery and animals but does not occur independently in the soil (Ostrofsky et al., 1977). Movement of infected Port-Orford-cedar nursery stock, including other Chamaecyparis species, also can spread the disease. Zobel et al. (1985) believe that the disease “probably never would have emerged in epidemic form without the widespread planting of ornamental Chamaecyparis in northwestern Oregon and Washington.”

The USDA Forest Service at the Oregon Dorena Tree Improvement Center, in conjunction with the USDI Bureau of Land Management and Oregon State University, have been working many years on the detection of resistance to this disease. Over 9,000 trees have been screened for resistance, and controlled pollinations are now being made to develop resistant genotypes and to understand the mechanisms of resistance. Meanwhile, surviving trees and stands are being protected by management practices, including restricted movement of people and machinery during wet periods where spores are abundant.